Imaging examination methods enable a physician or radiologist to diagnose a multiplicity of patient diseases. Many diseases require special examination techniques in order to ensure a reliable diagnosis. In particular in the treatment of vascular diseases, but also in tumor treatment, imaging diagnostics is becoming increasingly important. In the diagnosis and treatment of arteriosclerosis, for example, it is desirable to be able to quantify the plaques occurring in a patient's vascular system and leading to arteriosclerosis. Treatment is possible for example by means of diet or the use of cholesterol-lowering medicines. Until now it has only been possible to monitor individual plaques and their variation over time.
Positron emission tomography (PET) is becoming increasingly widely established alongside magnetic resonance tomography (MR) in medical diagnostics. While MR is an imaging method for representing structures and slices inside the body, PET allows in vivo visualization and quantification of metabolic activities.
PET uses the special properties of positron emitters and positron annihilation in order to quantitatively determine the function of organs or cell regions. With this technique, appropriate radiopharmaceuticals marked with radionuclides are administered to the patient prior to the examination. As they decay, the radionuclides emit positrons which after a short distance interact with an electron, causing what is termed annihilation to occur. This results in two gamma quanta which fly apart in opposite directions (offset by 180°). The gamma quanta are detected by two opposing PET detector modules within a specific time window (coincidence measurement), as a result of which the annihilation site is localized to a position on the line connecting said two detector modules.
In the case of PET, the detector module must generally cover a greater part of the gantry arc length for the purpose of detection. It is subdivided into detector elements having a side length of a few millimeters. On detecting a gamma quantum, each detector element generates an event record that specifies the time and the detection location, i.e. the corresponding detector element. This information is passed to a fast logic unit and compared. If two events coincide within a maximum time interval, it is assumed that a gamma decay process is taking place on the connecting line between the two associated detector elements. The PET image is reconstructed using a tomography algorithm, i.e. so-called back-projection.
Recorded PET images can be used for diagnosing plaques. According to a known method, PET images of a bodily region are recorded for that purpose using the tracer 18F fluorodeoxyglucose (FDG) and sites with increased metabolism, so-called “hot spots”, are identified. Because plaques or, as the case may be, inflamed vascular walls are characterized by increased metabolism, they become visible in this way in the recorded PET image. However, there are other phenomena, such as tumors, for example, which can also lead to an increased metabolism and therefore are likewise visible in the recorded PET image.
On the basis of the recorded PET image it is therefore hardly possible to decide which hot spots belong to a vascular wall, that is to say to a plaque, and which are attributable to other diseases. In order to support the interpretation of the recorded PET images, in many cases CT images of the same region are recorded in which calcifications, commonly referred to as “hard” plaques, are represented. If the calcifications coincide with a hot spot from the recorded PET image, the increased metabolism can easily be attributed anatomically with the aid of the recorded CT image. That said, however, it is only in a small number of cases that calcifications occur simultaneously with inflammations at the same point of the vascular wall, so assigning the hot spots to vascular walls is possible only in a few instances.
Furthermore, with known methods the assignment must be carried out manually, which process is both error-prone and time-consuming. In addition, the composition of so-called “soft” plaques cannot be determined by means of CT analyses. Soft plaques can be, for example, fat deposits, thrombi, connective tissue or tissue capsules. Nowadays, said types of soft plaques are readily distinguishable using established magnetic resonance tomography methods. Typing soft plaques is essential for the diagnosis, since it is precisely these soft plaques which are generally vulnerable and can lead to infarctions or embolisms.